What is that peak?

All engines produce some vibration as a result of power production. Recently it has become common for an X/Y vibration plot to be part of a dynamic propeller balance on reciprocating powerplants. Each engine produces a characteristic "signature" during operation. This signature can be valuable for comparison to other engines of the same type in like aircraft. They can also be compared by type to get an idea of what "normal" is. Since there are no published limits for levels of vibration on aircraft reciprocating engines this is virtually the only way to use the signature as a diagnostic tool in this application.

All 4 cycle engines produce vibration beginning at half engine speed (called half order - an order being a ratio to the crank). Since every other revolution of a four cycle engine is the power cycle, it occurs at half crank speed. A single cylinder 4 cycle motor produces half order. As cylinders are added, these half order events overlap to form combinations which produce other peaks up scale. Because of their physical distribution and individual uniqueness, it is difficult to pinpoint exact discrepancies on a given cylinder. If the propeller (or crank) is out of balance, there is a first order (order 1.0) vibration peak. This occurs at the crank RPM on direct drive engines. This is one of the easiest sources of vibration to diagnose and correct.

The remaining peaks occur at roughly half order intervals, so there will next be 1.5 order, 2.0 order and so on well through 5th order. Generally there are only three or four major peaks of interest.

If a peak is higher on the Y scale than other engines of the same type it is often an indicator of a potential problem. By well outside, we mean 200-300 % higher. The peaks can vary a little day to day and in fact run to run in some cases. These are small excursions though, typically no more than 10%. Fouling a plug on a hot start for instance will change the spectrum.

After a dynamic balance, the running condition of the crankshaft has changed. Since there is a change, the vibration peaks often change as well. It is common in counterweighted engines to have the 4.5 or 5th order increase immediately after a dynamic balance, only to show lower than the pre balance levels after only a few hours run time. In a similar vein, engines that have run in good dynamic balance for long periods generally have very low amplitude peaks across the board. There is a long term beneficial affect resulting from a true running crankshaft.

On the low end of the scale there are often large amplitude peaks in the 120 RPM range. Much of this is due to the bucking of the airplane on the ground as it lunges on the brakes. It has not yielded satisfactory engine or propeller data.

If a vibration is objectionable in the cabin, a cabin signature can be run to identify the frequency, and hence harmonic, of interest. In most light aircraft, the half order is most problematic. The mounts will often block all the high frequency peaks above 7000 RPM but the low end around half engine speed is not. Nor is blade pass rate (number of prop blades times the RPM), the rate at which the blade pulses act on the windshield and airframe. Worn engine isolators are often a culprit, although some factory recommended isolators are less than optimal.

All signatures furnished for logbook entries are looked at for possible discrepancies. As some of you know, I won't raise a flag unless there is compelling evidence of a real problem. Even if there is no problem today, the record is a valuable resource in case there is a problem in the future. The technique then used is called trend analysis. Trend analysis can catch incipient failures well in advance if it is done often enough. Two slices on the timeline are many more times valuable than one at any rate.

On fixed pitch propellers, you may notice a slight increase in static RPM. For a prop in the 150 HP range, 20-30 RPM is the norm. On propellers over .7 IPS, as much as a 75 RPM increase is sometimes seen. This translates into horsepower freed by virtue of a more efficient engine. It takes horsepower to swing an out of balance prop! On average, this translates into 1 or 2 extra horsepower. In flight testing has shown a slight increase in airspeed as a result, not to mention smoother operation.

On constant speed propellers, balance translates into less manifold pressure required for a given RPM. Obviously, this means less fuel consumed. Again more efficient operation. Another benefit is the ability to operate at lower RPM's where the engine is more efficient. Most engine isolators are very good at isolating higher frequency vibration but have difficulty in the lower speed ranges like half order and first order. With a balanced prop, you may find 2200 RPM more pleasant than previously used higher RPM settings.

Jim Fackler
JF Dynamics
(626) 358-7568
jfackler@earthlink.net

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Here are the results of the prop balance I had done on my stock YAK-52

Frank,
Attached are the hard copy records for your aircraft. Attach 1 is the initial idle reading. Attach 2 is the 60% reading where the balance was initially roughed in.

Attach 3 is the 75-80% readings showing the incremental progress on the prop. Since we did not have an initial 80% reading in the as is condition, I took the liberty of estimating the level. This is a conservative estimate based on the increase as % increased.

The two other plots are the balance polar plot (circular graph). This is actually the most reliable for final prop reading since the filter is automatically centered on the prop frequency. The other spectrum (in blue) is the digital spectrum. As you can see the readings do match very well to the analog instrument and polar record. Ah, the new digital age...

The vertical line on the Analog record (attach 3) at the prop frequency shows how much amplitude variation there is at the prop frequency. This is common for this type installation. Based on the data you are comfotably around .1 IPS velocity, .2 being considered acceptable.

Tha last two scratchy records are of Robins Yak. I used to mount on the cooling louvers and the record showed all the rattling going on there. You can still balance but you lose some clarity in the overall record. Note your peaks are roughly similar and Robin's airplane has been running well so you can expect the same.

Lastly, thanks for the kudo's. I've been contacted by a couple Yak owners. As I mentioned, the hardest thing for me is following bad acts. Apparently there are some out there. Actually I do check the blade angles, but I do it as part of the testing. Mine is a dynamic check. Last year I rejected three propellers for excessive blade angle variation and was 100% correct - never picked up a protractor on those.

The care and feeding you might find useful and the explanation of the FFT. It's for laymen of course, but helps them understand a little about what's going on...

Pleasure working with you Frank -

Jim Fackler
JF Dynamics
(626) 358-7568
jfackler@earthlink.net

PS - (Editor) This is the person that can diagnose a bent rod in your engine........

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The following is per Dennis Saverese..................................................

One extremely important check which must be done on the blades before any dynamic balance is performed is to verify the blade angles. I'll explain in detail how this can be done on the aircraft shortly.

I recently had my prop dynamically balance on my YAK 52. Before this was done, all the prop hub weights were on one side. After dynamically balancing the prop, we had to add more weight to the same side of the hub. The vibration was reduced dramatically though. We balanced the prop while running at 70%. Now here's the catch.

On my airplane there has always been a resonant vibration between about 46-48% and the high 50% area. I was told this is relatively common and is usually caused by mismatched blade angles and don't trust the "tic" marks on the blades or counterweights to set the proper blade angle. So dynamically balancing the prop without first accurately setting the blade angle on both blades only masks the root of the problem. My blades had a 2 degree difference between the blade angles. This is where the problem stemmed from.

The correct setting for the blade angle is 14.5 degrees, 8" in from the tip of the prop blade. Some of the props have a vertical line (if the blade is horizontal of course) marked on them. Usually they are the white ones. This line is typically the 8 inch line and is used to set the blade angle. You will need a smart level which measures angles electronically and has a digital readout in tenths of a degree.

Place the prop in a horizontal position. Next remove the hub spinner by removing the cotter pin and nut. Then remove the piston from the center of the hub being careful not to spill the oil into the hub unless you want oil spray all over the place for the first couple of hours after the next start (remember, we have a dry hub). Using the counterweights, you can now move the blades back and forth quite freely. Now you have a machined surface (the front of the prop hub) to use as a reference point. Place the smart level on the front of the hub and measure the angle. This is the airplanes angle of incidence. Make note of the angle. Now go to the prop blade on the left side as you face the airplane. Measure 8" in from the tip in two places approximately 1" on either side of the approximate center of the tip and make a vertical line across the blade. Be sure the blade is up against it's mechanical stop in the flat position. Next measure the blade angle using the smart level at this line CENTERING the smart level over the line. Make note of the blade angle. On mine, this is what I saw on the first blade. Again, my blades had a 2 degree difference between them. NOT acceptable.

Angle of incidence - 2.7 degrees
Blade angle - 79.3 degrees

Doing the math, 90 degrees minus 79.3 degrees = 10.7 degrees PLUS 2.7 degrees (A of I) = 13.4 degrees blade angle

Now rotate the prop 180 degrees and take the same measurements on the other blade. Don't forget to measure the A of I again after rotating the prop. The reference point is obviously very important and by rotating the prop or moving the airplane, you could change the A of I. So be sure to measure it each time for each blade.

To make the blade adjustment, at the rear of the counterweight is a 17 or 19 mm (can't remember which size) nut with a cotter pin thru it and the bolt. Loosen the nut so there is no clamping tension on the blade shank. Now gently rotate the left blade to achieve the 14.5 degree blade angle. Check your readings a couple of times and don't forget to set the calibration on the level beforehand for each blade. When the angle equals approx. 14.5 degrees, tighten the counterweight clamp. DO NOT reinstall a cotter pin at this time because you will want to recheck all your settings before you do put the new cotter pin in place. Do the same for the other blade.

After making the adjustment on both blades, recheck the settings again by rotating the prop and measuring the angle of incidence each time before measuring the blade angle. Reinstall the piston without any oil in it (because you WILL spill oil into the hub when you reinstall it), piston lock plate and screw, spinner, nut and cotter pin. Don't forget to safety wire the piston locking plate with the screw that you had to remove to be able to remove the piston in the first place.

When this is finished, then you can dynamically balance the blades. But be sure to start with no weights on the hub though.

Hope this helps.
Dennis Savarese
YAK 52 N152JB CLW

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Care and feeding of your Dynamically Balanced Propeller

Your propeller has been balanced to a level exceeding the manufacturer's most stringent specifications. Keeping it that way will result in many hours of safe and comfortable flying. The following guidelines are presented to inform owners and operators what should be done to maintain it's current state and what things will nullify that state.

Propeller Dressing
This may be done with some degree of impunity so long as the material is removed evenly and equally from all blades. Nicks deeper than 1/8 inch that are dressed over 3 inches should be rechecked for dynamic balance, especially if the nick is near the tip. Dents and gouges on the face (flat black side) may be smoothed without a rebalance.

Painting
Even painting of both blades is normally OK. Be especially careful near the tips. Put an equal number of coats on each blade. DO NOT strip the blade before painting. Some propellers are statically balanced by applications of paint and some propellers are dynamically balanced using this method. Depending on the propeller, you may end up with a very poor balance. If you want to touch up the black areas, it is best to rough up the surface with Scotchbrite and acetone and apply a light coat of flat black to both blades evenly. Better yet, consult your propeller manufacturer's guidelines.

Polishing
Unless you are polishing every day, three times a day, there isn't much chance you will adversely affect the balance to a significant degree. If you have stripped your blades to polish them see "Painting" above.

Balance interval
Most props are good for about 400 hours or 4 years between balances. For tail draggers the hour interval is higher, about 700 hours, because they are not as prone to foreign object damage. Amphibs and floatplanes should be checked every 200 hours if any significant time is spent on the water.

De-ice boot replacement and prop overhaul require a rebalance. At overhaul all previous dynamic balance weights should be removed.

Engine maintenance
Unless it's a new crankshaft, this will not usually affect the propeller balance. Be sure the balance weights and propeller are installed in the same relationship to the crank as they were when the balance was done.

Prop remove and reinstall
See Engine maintenance above.

With just a little care, your balance will be maintained and your engine will benefit throughout its life.